Nitration of aromatic compounds produces many large-volume chemicals such as nitrobenzene, nitrotoluenes, nitroxylenes, nitrochlorobenzenes and other nitroaromatics. Compounds such as these are important precursors and intermediates for aromatic diamines, explosives, dyes, pharmaceuticals, perfumes, pesticides, herbicides (such as pre-emergent herbicides), fiber systems, and many specialty chemicals.
Nitration of aromatic hydrocarbons has typically been carried out with a mixture of nitric and sulfuric acids in the liquid phase. However, the reactions are not very selective, which is particularly a problem if the para isomer is the more commercially desired isomer (e.g., if 4-nitro-o-xylene is more desired than 3-nitro-o-xylene). Conventional nitration processes often lead to over-nitration or to oxidized byproducts. In addition, aqueous work-up is often required, leading to the generation of large amounts of dilute aqueous acid waste streams, which cannot be recycled. This in turn, requires expensive and extensive separation and purification steps and disposal of waste.
In the past several decades, there have thus been concerted efforts in the agrichemical, specialties, and pharmaceutical industries to overcome such problems by developing cleaner, safer and/or more efficacious nitration processes. Such efforts have included processes that use, for example,                mixed acids that contain sulfuric acid, acetic anhydride, polyphosphoric acid (as discussed in JP-B-47/047,370);        solid acid catalysts such as zeolites (as discussed in Manoranjan et al, IN-A-2001DE01308); and        solid superacid SO42-/ZrO2 supported on mesoporous molecular sieve MCM-48 [as discussed in Xi et al, Huagong Jinzhan, 25(12), 1419-1422 (2006)].U.S. Pat. No. 6,376,726 discloses a single acid process for preparing nitroaromatic compounds in which aromatic hydrocarbons are nitrated in the liquid phase using fuming nitric acid in the presence of a metal ion-exchanged clay catalyst, wherein the metal ion is La3+, Cu2+ or Fe3+.        
Unfortunately, processes such as those mentioned above are characterized by low conversion, low regioselectivity to 4-nitro-o-xylene, or the formation of large amounts of undesired byproduct, to a large enough extent and/or with a large enough frequency that their commercial value is diminished. In U.S. Pat. No. 6,376,726, for example, in the nitration of o-xylene in Example 10 (Table 3), the highest selectivity of 4-nitro-o-xylene over 3-nitro-o-xylene reported is 53 to 47 at a conversion of 56.8%.
A need thus remains for an environmentally friendly, commercially viable process for the selective nitration of aromatic compounds at high conversion.